This invention relates to substances suitable for coating substrates, particularly flexible substrates such as textile sheet products, composed of (A)(a) a polyurethane containing hydrophilic groups or (A)(b) a precursor capable of forming a polyurethane (A)(a) at an elevated temperature and containing (i) blocked isocyanate prepolymer and (ii) cross-linking agents, and optionally (B)organic solvents. The invention further relates to the use of these substances for the preparation of coatings that are permeable to water vapor.
The term "polyurethanes" for the purpose of this invention also includes polyurethane ureas.
Coatings having a high permeability to water vapor have in the past repeatedly been the object of investigations and developments as they alone are suitable, for example, for the production of high quality leather substitutes or for articles of clothing with high wearing comfort.
Coatings of this kind may be produced by various methods, the most important of which will briefly be outlined here. A process which enables micropores to be produced in a coating by physical means has been known for a considerable time. In this process, a polymer (by which is meant, here and in the following, mainly a polyurethane polymer) dissolved in a solvent is applied in layers to a substrate or a separating support and transferred while still wet into a bath which contains a liquid that is a non-solvent for the polymer but is miscible with the solvent. The non-solvent therefore penetrates the layer of solution and gradually precipitates the polymer therein. When the resulting solidified film is dried, the solvent which escapes as well as the non-solvent leave microchannels behind which are available for the transport of water vapor in the coating.
It is similarly possible by the addition of powders of water-soluble salts to polymer solutions to produce coatings from which the salt can be washed out with water, leaving behind microcavities.
The perforation of compact polymer films by means of high energy electron radiation also gives rise to films that have high permeability to water vapor and can be laminated. This method, however, requires very expensive apparatus.
a variation of the microporous coating technique that dispenses with the complicated immersion bath technology but produces substantially the same results has recently been carried out. In this process--also known as "evaporation coagulation"--sufficient water is added to the solution of the polymer in a low boiling organic solvent so that the resultant spreadable paste still has just sufficient stability and can be applied. The organic solvent is the first component to be evaporated off. The water (which therefore continuously increases in proportion) precipitates the polymer as in the immersion process and finally escapes during drying., leaving a microporous structure in the film.
The immersion bath processes have the same major disadvantage as evaporation coagulation in that microchannels or microcavities weaken the permeable coatings. Consequently, the mechanical tensile strength and abrasion resistance are considerably inferior to those of a solid film. Moreover, the imperviousness to water is not always sufficiently high for some purposes, for example, rainwear.
No shortage of attempts have, therefore, been made to produce permeability to water vapor in a coating not only by use of physical methods but also by chemical means. Thus, for example, it has been proposed to produce such coatings with polyurethanes consisting partly of water-soluble or hydrophilic starting components.
Thus, German Offenlegungsschriften 1,220,384 and 1,226,071 describe polyurethane coating compositions in which polyurethanes have been prepared from glycols, diisocyantes, and polyethylene glycols having molecular weights of 1000.
Polyethlene glycols may also be used as diol components in polyester polyols for polyurethane elastomers for the purpose, as disclosed in Japanese Patent Application 61/009,423, of producing coatings which are permeable to water vapor and have little tendency to sell in water.
Solid top coats on composite materials of textile substrates and microporous coatings such as described in German Offenlegungsschrift 2,020,153 are also permeable to water vapor.
Segmented polyurethane elastomers of polyethylene glycols have also been disclosed in European Paten Application 52,915.
Other organic hydrophilic components have also been added to polyurethanes to produce water vapor-permeable composite materials and coatings. In particular, poly-.gamma.-methylglutamate may be added to polyurethanes, used either as starting components or grafted to the polyurethanes. The numerous literature references describing this use include German Offenlegungsschriften 1,022,329 and 1,949,060 and Japanese Patent Applications 58/057,420 and 59/036,781 are among the many literature references describing this use.
Polyurethanes containing the above-mentioned polyethylene glycols as starting components have very recently been the main object of technical interest in the field of solid coatings that are permeable to water vapor. These raw materials are inexpensive, widely available, and technically easily obtainable. The polyurethanes and polyurethane ureas obtainable from them are in principle well known. In contrast to the widely used polyurethanes and polyurethane ureas that contain polyester diols, polycarbonate diols, or polyether diols as relatively high molecular weight diols, the above-mentioned polyurethanes are water absorbent and permeable to water vapor and in some cases can even swell strongly or dissolve in water. Hydrophobic polyols are therefore added to the polyethylene glycols which cause the hydrophilic character. The resultant mixtures may be used to produce polyurethanes or polyurethane ureas that combine good permeability to water vapor with high resistance to the influences of liquid water.
Because sheet products produced from such polyurethanes are invariably exposed to a considerable amount of atmospheric moisture in natural surrounds and by their nature also store more water than conventional polymer films that are impervious to water vapor, the starting materials mentioned above must be exceptionally resistant to hydrolysis.
Hydrophilic units of relatively high molecular weight polyethylene glycols within the polyurethane chain improve the permeability of the coatings to water vapor. The capacity of these layers to swell in water, however, causes pustular swellings to appear on the coatings when discrete water droplets are placed on them. These pustules are not only aesthetically a disadvantage in a textile or leather article of use but also constitute a serious technical defect. The tendency of the coatings to swell also leads, for example, to insufficient wet fastness, insufficient resistance to abrasion in the wet state, and insufficient resistance to washing.
In practice, the proportion of polyethylene glycol having a molecular weight of 1000 or more must be sufficiently low to avoid the above-mentioned disadvantages, but it is then impossible to obtain a very high permeability to water vapor. The use of polyethylene glycols having a molecular weight less than about 1000 would be preferable for obtaining desirable properties but in the products known in the present state of the art this would dramatically reduce the permeability of the coating to water vapor.
It was now surprisingly found that coatings which are extremely permeable to water vapor and at the same time have a very low tendency to sell in water can be produced by using coating compositions consisting of systems which contain (A) a polyurethane having ionic groups and polyethylene oxide units that are defined according to quantity and the length of sequence or corresponding precursor containing blocked isocyanate prepolymers and a cross-linking agent and, optionally, (B) organic solvents for component (A).